Power Plant Engineering
In natural uranium, the constituents of three naturally occurring isotopes are

U₂₃₄ = 0.006%, U₂₃₅ = 99.282% and U₂₃₈ = 0.712%
U₂₃₄ = 0.712%, U₂₃₅ = 0.006% and U₂₃₈ = 99.282%
U₂₃₄ = 0.006%, U₂₃₅ = 0.712% and U₂₃₈ = 99.282%
U₂₃₄ = 99.282%, U₂₃₅ = 0.006% and U₂₃₈ = 0.712%

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Power Plant Engineering
A nuclear unit becoming critical means

Chain reaction that causes automatic splitting of the fuel nuclei has been established
It is capable of generating much more than rated capacity
There is danger of nuclear spread
It is generating power to rated capacity

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